1. Field of the Invention
The present invention relates generally to receivers for angularly modulated signals. More particularly, the present invention relates to apparatus and method for increasing the sensitivity of both FM and PM receivers.
2. Description of the Related Art
In radio receivers, one of two systems for modulating the signal has generally been used: amplitude modulation, in which the amplitude of the carrier is varied, and frequency modulation, in which the frequency of the carrier is varied.
Frequency modulation is also called angular modulation, because modulation of the carrier frequency results in angular deviations in the carrier frequency, or center frequency. Also included in the category of angular modulation is phase modulation.
Phase modulation differs from frequency modulation in that the phase of the carrier frequency is varied directly, rather than directly varying the frequency of the carrier. However, phase modulation also varies the frequency of the carrier, and frequency modulation also varies the phase of the carrier. In a receiver, an FM demodulator is equal to a PM demodulator plus a differentiator, and a PM demodulator is equal to an FM demodulator plus an integrator.
Frequency modulated transmitters and receivers have been used widely, the applications including relatively unsophisticated audio radios for receiving commercial broadcasts, receiving the audio portion of consumer video, personal FM communications, amateur radio, industrial radio uses that include audio, digital data, and video, and military communications that include audio, digital data, and video.
PM transmission has enjoyed less popularity than FM transmission. The reason for this is: the phase of a carrier varies as a function of the distance between the transmitter and the receiver, and also atmospheric conditions. Therefore, when transmitting digital information, the phase of the received signal will change 180 degrees whenever the distance between the transmitter and receiver changes by a half wave length, and so, for digital communications, it becomes difficult to correlate the phase modulation with the phase of the unmodulated carrier. While various methods have been used, either at the transmitter or at the receiver, to overcome this problem, FM communications have enjoyed more popularity than PM communications.
The bandwidth required for angular modulation to pass all significant sidebands is equal to twice the product of the highest modulating signal and the number of significant sidebands as determined from the table of Bessel functions. However, the required bandwidth can be approximated by Carson's rule which states that the required bandwidth is equal to twice the sum of the peak frequency deviation and the highest modulating signal frequency.
A primary objective with any radio-frequency link is to achieve reliable communications with the least transmitter power, or to achieve reliable communications for the greatest distance. Obviously, more reliable communications over greater distances can be achieved with the same transmitter power, and with the same antenna size and complexity, if the sensitivity of the receiver can be increased.
However, atmospheric noise is nearly constant across the entire band width, and the required predetection bandwidth in all FM and PM receivers has been a function of the peak frequency deviation and the highest modulating frequency, in accordance with Carson's rule. Therefore, with a minimum and irreducible required bandwidth, it has been almost impossible to increase the signal noise ratio beyond previously achieved levels. That is, it has been almost impossible to achieve any noticeable gains in the sensitivity of FM and PM receivers.
Typically, FM and PM receivers have included: an RF input stage having a preselector and an amplifier; an IF stage having a first mixer and a first local oscillator; a second IF stage having an IF amplifier/filter, a second mixer, and a second local oscillator; and a demodulator.
Various types of demodulators, or discriminators, have been used in FM and PM receivers. However, since the advent of integrated-chip electronics, demodulators which use tuned circuits have come into disfavor because their cost is greater than demodulators which consist only of an integrated chip. Some of the demodulators which are adaptable to integrated circuits and which are presently popular include: the pulse averaging demodulator, the quadrature detector, and the phase-locked loop demodulator.
As noted above, phase-locked loop oscillators may be used in the frequency converter of the angularly modulated receiver. The discussion which follows pertains to D.C. modulated phase-locked oscillators.
The frequency of radio frequency voltage controlled oscillators (RF VCO) has been closely controlled by phase locking a feedback signal from the RF VCO to a crystal controlled reference oscillator (XO). A phase detector has been used to determine the phase difference between the feedback signal and a crystal controlled reference frequency; and an integrator has been used to summate the phase difference and to control the frequency of the RF VCO oscillator in accordance with the summated phase difference.
Improvements taught by the prior art over the basic phase locked oscillator include the use of prescalers to provide a feedback signal having a lower frequency than the RF VCO, thereby lowering the required frequency of the controlling circuitry. Prior-art improvements over the basic circuitry also include the use of a dual modulus divider to channelize the output frequency by dividing the feedback by higher and lower dividing ratios in a technique known as pulse swallowing. That is, channelizing is accomplished by swallowing, or removing, pulses in the feedback path.